1. Field of the Invention
This invention relates generally to a method and apparatus for the magnetic recording of digital video signals and to a magnetic recording medium for use therein.
2. Description of the Prior Art
A D1 format component type digital VTR and a D2 format composite type digital VTR have been developed for use by broadcasting stations in digitizing color video signals and recording the digitized signals on a recording medium, such as a magnetic tape.
In the D1 format digital VTR, a luminance signal and first and second color difference signals are A/D converted with sampling frequencies of 13.5 MHz and 6.75 MHz, respectively. Thereafter, the signals are suitably processed and then recorded on a tape. Since the ratio of sampling frequencies of the signal components is 4:2:2, this system is usually referred to as the 4:2:2 system.
On the other hand, in the D2 format video digital VTR, a composite video signal is sampled with a signal having a frequency 4 times higher than the frequency fsc of a color subcarrier signal and then A/D converted. Thereafter, the resultant signal is suitably processed and then recorded on a magnetic tape.
Since these known D1 and D2 format digital VTRs are designed for professional use, for example, in broadcasting stations, the attainment of high picture quality is given top priority in the design and construction of such VTRs, and the weight and size of the apparatus are not overly important.
In these known digital VTRs, the digital color video signal, which results from each sample being A/D converted into, for example, 8 bits, is recorded without being substantially compressed. As an example, when the known D1 format digital VTR A/D converts each sample into 8 bits with the frequencies noted above, the data rate representing the color video signal is approximately 216 Mbps (megabits per second). When the data in the horizontal and vertical blanking intervals are removed, the number of effective picture elements of the luminance signal per horizontal interval and the number of effective picture elements of each color difference signal per horizontal interval become 720 and 360, respectively. In the NTSC system which has 525 lines per frame and 60 fields per second, the number of effective scanning lines for each field is 250, and the data bit rate Dv can be expressed as follows: EQU Dv=(720+360+360).times.8.times.250.times.60=172.8 Mbps
Even in the PAL system, since the number of effective scanning lines for each field is 300 and the number of fields per second is 50, it is obvious that the data bit rate in the PAL system is the same as that in the NTSC system. If the redundant components necessary for error correction and the format with respect to such data are considered, the total bit rate of picture data becomes approximately 205.8 Mbps.
Further, the amount of audio data Da is approximately 12.8 Mbps, while the amount of additional data Do, such as, data of a gap, a preamble, and a postamble used in editing, is approximately 6.6 Mbps. Thus, the bit rate of information data to be recorded can be expressed by the following equation: EQU Dt=Dv+Da+Do EQU Dt=172.8+12.8+6.6=192.2 Mbps.
In order to record such amount of information data, the known D1 format digital VTR employs a segment system having a track pattern comprised of 10 tracks for each field in the NTSC system, or comprised of 12 tracks for each field in the PAL system.
In the known D1 format digital VTRs, a recording tape with a width of 19 mm is used. There are two types of recording tapes having thicknesses of 13 .mu.m and 16 .mu.m, respectively. To house these tapes, there are three types of cassettes, which are respectively characterized as being of the large size, middle size, and small size. The information data is recorded on such tapes in the above mentioned format with the tape area for each bit of data being approximately 20.4 .mu.m.sup.2 /bit, which corresponds to a recording density of 1/20.4 bit/.mu.m.sup.2. When the recording density is increased, an error tends to take place in the playback output data due to interference between codes or nonlinearity of the electromagnetic conversion system of the head and tape. Heretofore, even if error correction encoding has been performed, the above given value of the recording density has been the limit therefor.
By putting all the above described parameters together, the playback times for the cassettes of various sizes and the two tape thicknesses, when employed in the digital VTR in the D1 format can be tabulated as follows:
______________________________________ Size/tape thickness 13 .mu.m 16 .mu.m ______________________________________ Small 13 minutes 11 minutes Middle 42 minutes 34 minutes Large 94 minutes 76 minutes ______________________________________
Although the described D1 format digital VTR can provide satisfactorily high picture quality for use in broadcasting stations, even if a large cassette housing a tape with a thickness of 13 .mu.m is used, the playback time is at most 1.5 hours. Thus, such a VTR is not adequate for consumer or home use in which a playback time at least sufficient for the 5 recording of a telecast movie is required. On the other hand, in VTRs intended for consumer or home use, the .beta. system, the VHS system, the 8-mm system, and so forth have been employed. However, in each of these systems for consumer or home use, analog signals have been recorded and reproduced. Although the picture quality of these analog VTRs has been improved to the point where the quality is satisfactory when a video signal is simply recorded and then reproduced for viewing, the picture quality is significantly degraded when the recorded signal is dubbed and copied. Thus, when the recorded signal is dubbed several times, the picture quality will become unacceptable to the viewers.
If signals of wave lengths as short as 0.5 .mu.m are recorded in tracks having a width of 5 .mu.m so as to achieve a recording area of 1.25 .mu.m.sup.2 per bit, that is, a recording density of 0.8 bit per .mu.m.sup.2, and if such recording operation is performed in conjunction with a suitable data compression which adequately compresses the recording data without substantial distortion of the reproduced signals, then long-timed digital recording and reproducing of video signals may be possible even with magnetic tapes having a width of 8 mm or less and a length that can be readily contained in a cassette of a size not substantially larger than the cassette used with the analog 8-mm system.
However, attempts to decrease the recorded bit area to about 1.25 .mu.m.sup.2 per bit have resulted in unacceptably high bit error rates upon playback.